Gravity meter



p l 8 94 'D. H. CLEWELL 2,281,001

GRAVITY METER Filed May 24, 1940 UNITED STATES PATENT OFFICE 2,281,001 GRAVITY METER Dayton H. ClewelL-Dallas, Tex, asslgnor, by mesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a. corporation of New York Application May 24, 1940, Serial No. 336,898

7 Claims.

This invention relates generally to gravity meters and more particularly to an apparatus for nulling such instruments.

Many types of gravity meters are known in the prior art. Some of these types have been used with a degree of success in conducting geophysical surveys. However, to date no one has produced a satisfactory system for nulling instruments of this character. This primarily due to the delicateness of the elements of the instrue ment itself and the small forces or variations in forces that must be measured.

Gravity meters which measure directly the force of gravity by observing the displacement of ing by effecting a shift in the center of gravity of the mass thereby changing the, effective lever arm through which the weight of the mass acts.

Therefore, it is the primary object of this invention to provide an apparatus for nulling a gravity meter of the horizontal beam type by shifting the center of gravity of the mass an amount suflicient to change the lever arm through which the weight of the mass acts that it will return to its zero or base station reading without altering the sensitivity of the device.

Another object of this invention resides in the I without applying any additional elastic forces to g the pivoted mass.

Still another object of this invention resides in the provision of a null system for gravity meters which utilizes a very small auxiliary mass that is pendulously supported by the main mass and means for applying a small force to the auxiliary mass to shift it horizontally.

a mass are inaccurate, for from day to day the her objec s nd a van w l c m pphysical characteristics of the delicate elements pa m the following detailed s p n. comprising the operating mechanism of the in when considered with the attached drawing, in strument vary; Such variations obviously rewhich: suit in a change in the sensitivity of the instru- Figure 1 is a tic ev t onal view of a ment to the. small forces or variations in force horizontal pivoted beam yp ty e er showthat it is'desired to measure. Since in conduct- 8' the apparatus by means 1 ch the mass ing reconnaissance geophysical surveys by means is returned to its zero reading or base station of the gravitationalmethod one is only interreading after d p e t; and ested in the variations in gravitational force from Figure 2 s a p v w of the instrument p y point t point r an area of t surface of the in section taken along the line 2-2 of Figure 1. earth, greater accuracy could be obtained by e- Referring to the drawing in detail and particsorting to a null system for measuring these ulerly to Figure 1, there is sh wn h m i l y v ri tion a gravity meter of the horizontal beam type In every instance it is desirable to null the incomprising a mess from which a am H exstrument without in any way afi'ecting t sensp tends horizontally to a pivot l2. Pivot I2 is tivity, for to change the sensitivity of the inmounted upene support 13 that is secured to t t while conducting a Survey of a Pattiethe case, not shown, of the gravity meter. The ular area would give erroneous results which composed of a heavy bar p ng l4 m t be comparable and could t be that is secured to the support l3 and to very light related to give a true contour map that would f Springs 55 f The heavy Sp ng M be a picture of the elevation of the substrata. being a rather stlfi flat Spring, Serves o ushion The problem then immediately confronting the tension on the leaf springs i5 d 16 that design engineer is to produce a null ystem might result from a small misalignment Of the means of which the mass after displacement can when the es s rigidly held in clamped be returned to its zero position or base station posltlon- The D S d I5 are q a v position by some means other than a variation on opposite sldes of the beam H and a e of the elastic forces acting upon the mass. To m parallel relatlon 1 each Other- The upper this end the present, invention'aifords a simple ends of the springs and are lf and effective method and apparatus for returning to the beam; Sprmg M at points near t opposlte the mass to its zero position or base station readends thereo The lower ends of Sprmgs and I6 are clamped in the clutches l1 and I8 re spectively, which are carried by the rod I9. The beam II is secured to the rod I9 by means of a clamp 20 which is formed integrally with the end of the beam or is rigidly secured thereto. The

clamp 20 is rigidly secured to the rod Why means of screws or bolts 2| so that movement of the beam II in a vertical plane will rotate the rod l9 and cause the springs l5 and It to flex. The pivotaLaxis thus formed for the beam lies along a transverse line drawn across the approximate centers of the light springs l5 and It.

In order to elastically equalize the gravitational forces acting upon the mass it, an arm 22 is secured to the mass and is adapted to extend substantially vertically therefrom'at an angle of 90 to the beam ii. The upper end of arm 22 is provided with anchoring means 23 to which one end of a tensioned coil spring 2a is secured. Spring 26 extends horizontally along a line that is substantially parallel to the beam H to a fixed anchorage 25 that is fastened to or a part of the gravity meter casing, not shown. Spring 23 is secured to the anchorage means 25 by means of an adjusting screw 26 which lies in axial alignment with the spring 24 and threadediy engages the anchorage means 25.

Since with devices of this character it is necessary' to measure forces within an accuracy of one part in ten million, very delicate elements which will retain their physical characteristics must be used. Therefore, in order to maintain the physical characteristics of the delicate elements of this device constant, it is necessary that means he provided to rigidly clamp the mass l0 while the instrument is being subjected to jars or shocks which would alter these physical characteristics. To this end support 21 having fingers 28 projecting from its face 29 is secured to the gravity meter casing and disposed adjacent the mass I0. Mass Ill is provided with opposite parallel faces 'that are substantially parallel with the face 29 of the support 21. The fingers 28 are so formed that they make point contacts with face-30 of the mass II] when the mass is forced into contact with them. On the opposite side of the mass from the support 2'! there is provided a movable plunger 3| having a point 32 that is adapted to engage face 33 of the mass Ill. The plunger 3| is slidably journaled within the cylindrical casing 34 and adapted to be forced outwardly and downwardly to contact themass In by means of the coil spring 35. One end of the coil spring rests against an annular shoulder 36 formed on the plunger 3|. The other end of the coil spring which is concentrically disposed about the stem 31 of the plunger 3| bears against a plug 38 which forms a closure for the cylindrical casing 34. Plug 38 is provided with a centrally disposed opening through which the stem 31 of the plunger 3| is adapted to project, it being understood that the stem 31 will slide freely in the opening in the plug 38. The plunger 3| is provided with pins 39 which extend from diametrically opposite sides through elongated openings 40 in the cylindrical casing 34. The pins 39 extend outside of the casing 34 farenough to engage the arms of a yoke 4|. The yoke 4| is pivotally secured at 42 to the casing 34. The arm 43 of the yoke 4| extends outwardly at such an angle that a rod 44 extending from a point outside of the gravity meter casing and carried by bearings 45 can contact the arm 43 to pivot the yoke and retract the plunger 3| carrying the point 32 out of contact with the mass Ill. It is obvious to those skilled in the art that this clamping means can be replaced by hydraulic clamping means or any other suitable means which would firmly secure the mass ||I against movement when the instrument is being moved from point to point.

Any suitable means known in the art can be used to measure the displacement of the mass II, as for example, an indicator 46 carried by the arm 22 that is adapted to cooperate with a scale the gravity meter set up at this point.

able and more feasible to use a null system formeasuring these extremely minute variations in force. For the most satisfactory results it is desirous to use a null system which will not alter the sensitivity of the instrument, that is, means which will return the mass to a zero or base station reading withoutvarying the sensitivity of the device. To this end applicant has pendulously suspended from the mass 10 a very small I mass by means of an extremely light and flexible spring 49. The small mass 48 is spaced from the mass II] adistance sufliciently far that by moving it horizontally in a direction parallel to the beam II, the effective center of gravity of the entire mass assembly can be shifted sufliciently far to null the instrument or bring it back to the zero or base station reading under.

any conditions encountered in measuring the variations in gravitational force. To effect a horizontal movement of the small mass 48 there is provided two light springs 50 and 5| that extend in axial alignment from opposite sides of the mass 48. The outer end of spring 50 is anchored to a fixed support 52 that is secured to the gravity meter casing, not shown, while the outer end of spring 5| is axially secured to an adjusting screw 53 which passes through and threadedly engages a support 54 that is secured to the gravity meter casing, not shown. Springs 50 and 5| are very light and normally under sufficient tension that a changein the adjustment of screw 53 will cause the mass 43 to shift in the desired direction.

In operation in conducting a geophysical survey of a particular area, a plurality of stations are located over this area in such a manner that they will afford substantially complete coverage. Then a base or reference station is selected and The mass is then unclamped and by means of the screw 26, shown in Figure 1, the mass and indicator 45 carried thereby is caused to move until the indicator travels to approximately the zero point on the scale ll. Further adjustment is then made of the position of the mass to bring the pointer 46 exactly to the zero mark on the scale M by horizontally shifting the mass 48 by means of the springs 50 and 5| and the adjusting screw 53. When this zero condition is attained, the instrument is clamped by causing the plunger 3| to ,move downwardly causing the point 32'carried thereby to press against the face 33 of the mass I0. Plunger 3| is allowed to force the mass down firmly against the pins 28 to securely clamp it against movement which would injure or change the physical characteristics of the delicate elements that support the mass when the instrument is being moved to a new'location.

After the mass has been securely clamped, the instrument is moved to a new location and properly set up at that point. Then the mass is released and the amount of horizontal displacement of the mass 48 required to bring the indicator 46 carried by the main mass back to the zero point on the scale 41 is read. The scale 55 which cooperates with the pointer 56 carried by the screw 53 can be calibrated in units of, gravity and readings made from this scale are proportional to variations in gravitationalforce between the base or reference station and the station under observation.

After the reading has been made at the-station, the mass is clamped in the manner described above and the process repeated until the entire area has been covered. The data. thus gathered is then plotted in the form of contour maps which simulate the elevation of the substrata. I

Many other detailed features that may be added as refinement to the g avity meter illustrated will immediately become apparent to those skilled in the art, and it is to be understood that thescope of this invention includes the principles of this invention regardless of whether or not these numerous additional features are incorporated.

I claim:

1. In a gravity meter that comprises a support,

a main mass, means for pivotally securing the massto the support in such a manner that .the

pivotal point and the center of gravity of the mass to the support in such a manner that the pivotal point and the center-of gravity of the mass are in different vertical planes, elastic means for supporting the mass against the action of vertical components of gravitational force, means for indicating the disposition of the mass relative to the support, and means for nulling the gravity meter by returning the mass to a pre determined indicated position, said means comprising an auxiliary mass pendulously fixed to the main mass, common means for shifting the auxiliary mass in a substantially horizontal direction to effect a change in the lever arm through whichthe center of gravity of the system formed by the main mass and the auxiliary mass acts, and for elastically holding the auxiliary mass in its shifted position.

5. In a gravity meter that comprises a support,

a main masss-means for pivotally securing the mass are in different vertical planes, elastic means for supporting the mass against the action of vertical components of gravitational force,

means for indicating the disposition of the mass relative to the support, and means for nulling to the main mass and means for shifting the auxiliary mass in a substantially horizontal direction to efiect a change in the location of the center of gravity through which gravitational forces act. i

2. In a gravity meter that comprises a support, a main mass, means for pivotally securing the mass to the support in sucha manner that the pivotal point and the center of gravity of the mass are in different vertical planes, elastic means for supporting the mass against the action of vertical components of gravitational force, means for indicating the disposition of the mass relative to the support, and means for nulling the gravity meter by returning the mass to a predetermined indicated position, said means comprising an auxiliary mass pendulously fixed to the main mass, and means for shifting the auxiliary mass in a substantially horizontal dithe gravity meter by returning the mass to a pre-' determined indicated position, said means comprising an auxiliary mass, elastic means for pendulously suspending the auxiliary mass from the main mass, and means for shifting the auxiliary mass relative to the main mass to effect a change in the lever arm through which the weight of the masses acts.

6. In a gravity meter that comprises a support, a main mass, means for pivotally securing the mass to the support in such a manner that the pivotal point and the center of gravity of the mass are in different vertical planes, elastic means for supporting the mass against the action of vertical components of gravitational force, means for indicating-the disposition of the mass relative to the support, and means for nulling the gravity meter by returning the mass to a predetermined indicated position, said means auxiliary mass relative to the main mass to-efrection to effect a change in the lever arm l through which the weight of the masses acts. 3. In a gravity meter that comprises a support, a main mass, means for pivotally securing the mass to the support in such a manner that the pivotal point and the center of gravity of the mass are in different vertical planes, elastic means for supporting the mass against the action of vertical components of gravitational force, means for indicating the disposition of the mass relative to the support, and means for nullmg the gravity meter by returning the mass to l a main mass, means for pivotally securing'the fact a change in the lever arm through which the weight of the masses acts, and for elastically holding the auxiliary mass in shifted position.

7. In a gravity meter that comprises a support, a main mass, means for pivotally securing the mass to the support in such a manner that the pivotal point and the center of gravity of the mass are in difierent vertical planes, elastic means for supporting the mass a ainst the action of vertical components of gravitational force, means for indicating the disposition of the mass relative to the support, and means for nulling the gravity meter by returning the mass to a predetermind indicated position, said means comprising an auxiliary mass, pendulously fixed to the main mass, means for shifting horizontally the position of the auxiliary mass relative to the main mass to effect a change in the center of gravity of the system formed by the main mass and the auxiliary mass, and serving as an elastic suspension for holding the auxiliary mass in its shifted position.

- DAYTON H. CLEWELL. 

